In the field of papermaking, the production of a sheet of paper begins at a headbox which contains a slurry of liquid and pulp containing paper forming fibers. The headbox has an elongated opening or slice lip through which the slurry under pressure is deposited onto a moving Fourdrinier wire or screen. The screen assists in separating the fibers from the liquid to create a web of material which is the initial step in the papermaking process.
At the headbox, the slurry is deposited onto the wire and travels in the machine direction (MD). A series of actuators arranged along the cross-direction (CD) of the papermaking machine (transverse to the machine direction) control locally the size of the slice opening to permit the passage of greater or lesser amounts of slurry from the opening. The headbox is the primary means for controlling the quality and grade of the paper being manufactured.
An important factor in controlling the quality and grade of the paper is monitoring the Fiber Orientation of fibers emerging from the headbox. Fiber Orientation (FO) is the term used to discuss how fibers lay horizontally within a sheet of paper or board. Identifying the direction in which the majority of fibers are aligned (Fiber Orientation Angle) and the degree of alignment (Fiber Ratio, Aspect Ratio, or Index), characterizes the Fiber Orientation. Fiber Orientation Angle is the direction the majority of the fibers are laying with respect to the machine direction. Fiber Ratio is a measurement of the anisotropy (exhibiting properties with different values when measured in different directions), or percentage of fibers not lying in the Fiber Orientation direction. The Aspect Ratio describes the relative numbers of fibers oriented with the Fiber Orientation Angle and perpendicular to the Fiber Orientation Angle. Undesirable Fiber Orientation can reduce paper runnability during printing and converting operations, causing such problems as curl, stack lean, twist warp, miss-registration, and others. Since Fiber Orientation is determined between the stock approach system at the headbox and the dry-line on the forming table at the Fourdrinier wire, potential “handles” for affecting Fiber Orientation are also found in this area of the machine.
In conventional arrangements, most of the headbox delivery system components are manually adjusted, such as headbox balance (re-circulation), manifold bellows, edge flows and cheek bleeds. Unbalanced headboxes can cause cross flows within the headbox which tend to align fibers detrimentally. The manifold bellows give some headboxes the ability to change the pressures or flows non-linearly across the box. Edge flows give the ability to control fiber angle using extra flows on the sides of the headbox. Cheek bleed removes stock off the sides of the headbox, or reverse bleed injects stock back into the headbox edges. Any modification of the “bleed” flows on the side of the headbox will significantly affect fiber angle. Most of the affect will be on the outside edges of the sheet where fiber angle is usually the largest problem. Hang-down or “stick” is the distance the slice lip hangs below the front wall, and has a significant effect on the turbulent flow of stock onto the breast roll. Additionally, the front wall can often be moved horizontally, as can the apron, which changes the impingement angle. Another adjustment for Fiber Orientation within many headboxes are rectifier rolls, which are drilled rolls that turn in various directions at various speeds to induce turbulence in the stock. Dilution flow control or Consistency Profiling and similar retrofit systems such as the BTF Distributor, affect basis weight discretely across the width of the machine, so with the use of a slice lip, it is possible to control the relative velocities independently from the basis weight. This allows both basis weight and Fiber Orientation to be simultaneously and independently optimized.
Current paper manufacturing machines often rely on measurement schemes that determine Fiber Orientation of the finished product. Measuring the Fiber Orientation of finished product (at the dry-end) has several problems. One problem is that different running conditions make it impossible to correctly control Fiber Orientation, since these varying conditions can change both the gain and its sign for the control. As the Fiber Index approaches one, where the sheet is described as “square”, dry-end measurements have a very difficult time determining the direction and magnitude of the Fiber Orientation. Additionally, dry-end measurements are only good for either bulk or at best top and bottom fiber orientation measurements, and cannot provide adequate information about middle layers in a multi-layer product. Separate Fiber Orientation information from the separate layers will also make it possible to repeat the same quality on different grade runs. It may also facilitate the development of new grades with improved properties.
In 1971, a system for measuring the velocity of a jet emerging from a head box in a paper manufacturing system was patented by Industrial Nucleonics Corporation (U.S. Pat. No. 3,620,914). This reference discloses that: “Jet velocity is determined by measuring the Doppler shift frequency caused by the jet on a laser beam of coherent electromagnetic energy. The velocity of the jet is compared with the velocity of a Fourdrinier wire which receives the jet, whereby there is derived a signal for enabling a predetermined relative velocity between the jet and the wire to be automatically or manually maintained. The laser beam is scanned across the width of the jet to determine differences in the jet velocity as a function of width.”
In 1989 Beloit Corporation received U.S. Pat. No. 4,856,895 directed to a method of measuring the jet velocity. The patent relies on measurement of the velocity of a liquid jet from the headbox. The patent states: “The velocity of a liquid jet, such as the headbox jet of a paper making machine, is measured by cross-correlation of a.c. signal components produced by a pair of light beams received by a pair of photodiodes. The light is supplied by a single source, an incandescent lamp, and is guided by a pair of bifurcated fiber optics mounted above the jet and spaced apart in the flow direction. The a.c. components are filtered to remove flow frequencies, amplified and then analyzed in a spectrum analyzer.”
In 1992, the Weyerhaeuser Company received U.S. Pat. No. 5,145,560 which is also directed to monitoring of headbox jet velocity. This reference discloses that: “The jet velocity along a slice opening of a papermaking machine is monitored at plural locations to provide a jet velocity profile. This jet velocity profile may be adjusted to more closely match a reference velocity profile for the jet. Preferably, microwave Doppler effect velocity sensors are utilized for sensing a jet velocity.”
In 2000, the Voith Paper Company received EP Patent No. EP 1116825A entitled “Method for Fiber Orientation Control”, which describes a method to measure and control a cross-machine velocity profile of a fibrous stock suspension jet at the outlet from the flow box nozzle.
In 2002, Honeywell International received U.S. Pat. No. 6,437,855 entitled “Laser Doppler Velocimeter With High Immunity to Phase Noise”. A true Doppler frequency is extracted from the phase noise frequencies by maintaining a highest frequency value. The highest frequency value is replaced with any measured frequency values that are higher than the current highest frequency value. This is continued for a predetermined lifetime period, after which the highest frequency value is stored and then reinitialized. The highest detected frequency values over a window of lifetimes are then averaged to provide a moving or rolling average value, which is indicative of the velocity of a medium.
Also in 2002, Stora Enso presented a paper at the SPCI 2002 Controls Conference entitled “Jet Misalignment, “The Missing Link” in Headbox Control is Now Available”, by Ulf Andersson, Research Engineer Packaging Board Stora Enso Research, Karlstad PO Box 9090 S-650 09 Karlstad, Sweden. This paper was based upon Swedish Patent No. 515640 issued Sep. 17, 2001 to Stora Kopparbergs Bergslags AB.